A. Field
The present invention relates to a method for selectively recovering a metal, groups of metals, and/or metal compound(s) from a metal-bearing raw source material containing chromium (Cr).
B. Related Art
Industrial, mining, and manufacturing processes generate large amounts of metal-bearing raw source material on daily basis. This metal-bearing raw source material includes mining ores, ore concentrates, waste products, residues and byproducts. Metal-bearing raw source material often contains valuable nonferrous metals such as chromium (Cr), nickel (Ni), copper (Cu), cobalt (Co), tin (Sn), zinc (Zn), molybdenum (Mo), manganese (Mn), lead (Pb), cadmium (Cd), vanadium (V), as well as precious and platinum group metals including silver (Ag), gold (Au), palladium (Pd), platinum (Pt), rhodium (Rh), ruthenium (Ru), osmium (Os), and iridium (Ir).
The disposal of metal-bearing raw source material containing these metals raises serious environmental and business concerns on a global level due to the hazardous nature, potential toxicity, and risk to human health posed by the presence of these metals. The costs associated with the disposal of hazardous metal-bearing raw source material in the absence of metal reclamation are enormous. In this regard, recovery of metals from metal-bearing raw source material not only would reduce the volume and cost of disposal, but the recovered metals could be resold or reused to provide substantial economic value. The expenses and environmental impact associated with disposing of metal-bearing raw source material, along with the economic value of the incorporated metals, has generated interest in how to treat and recover metals from metal-bearing raw source material.
Current methods for treating and recovering metals from metal-bearing raw source material, however, are often inefficient and expensive to implement. It has been particularly challenging to treat and recover metals from metal-bearing raw source material that contains Cr, as Cr is difficult to separate from other metals and metal compounds.
For example, vitrification is a proven technique in the disposal and long-term storage of nuclear waste. However, the presence of Cr dramatically increases the bulk of the nuclear waste. In order to economize and reduce the quantity of nuclear waste, the Cr content is hydrometallurgically separated and removed, thereby decreasing the total amount of nuclear waste to be vitrified. See Rapko et al., “Selective Leaching of Chromium from Hanford Tank Sludge 241-U-108”, Pacific Northwest National Laboratory, PNNL-14019, article prepared for the U.S. Department of Energy under Contract DE-AC06-76L01830. Rapko et al disclose that the Cr can be selectively leached from the nuclear waste through an oxidative alkaline leaching process. The process, however, utilizes expensive reactants and is not concerned with the recovery or economic value of other metals that might be present in the nuclear waste. The primary objective of Rapko et al. is to effectively reduce the vitrification cost by reducing the quantity of nuclear waste to be vitrified. The removal of the Cr component from the nuclear waste satisfies this objective by reducing the final quantity of waste that must be vitrified, thus lowering the overall cost of processing.
U.S. Pat. No. 5,200,088 describes a process for removing hexavalent chromium (Cr(VI)) from a waste product. This patent suggests that the most hazardous form of chromium is Cr(VI) and that the presence of Cr(VI) in the waste product must be reduced to a few parts per million (ppm) or less before the waste product can be discarded. In accordance with the process described in this patent, the Cr(VI) in the waste product is converted by treating the waste product with an alkali metal dithionite to reduce the Cr(VI) to trivalent chromium (Cr (III)). The result is a soluble material that forms a precipitate at reduced pH. The precipitate containing the Cr(III) can then be separated from the remaining waste product. However, the patent does not reveal any interest in the recovery or separation of other metals that may be present in the Cr-bearing waste.
U.S. Pat. No. 4,162,294 describes a method for recovering Cr and at least one other metal from a metal-bearing raw source material containing Cr. In particular, the method involves chlorinating a waste sludge containing Cr, aluminum (Al), Cu, Zn, and Ni to oxidize the Cr into a soluble form and to obtain an insoluble component that contains the Al, Cu, Zn, and Ni; separating the Cr in soluble form from the insoluble component with a fixed bed anion exchanger; and separating the Al, Cu, Zn, and Ni present in the insoluble component through an elaborate series of liquid-liquid extractions and precipitation steps.
However, ion exchange is relatively costly, slow, and cumbersome to use. In order to be effective, the Cr-bearing material being treated must be passed through a significant amount of ion-exchange resin, usually in the form of a filter bed, making it effective, in most cases, for treating only small volumes of wastewater. Thus, ion exchange would be impractical as an initial step for separating metals from complex metal-bearing raw source material. Furthermore, the series of liquid-liquid extractions and precipitation steps is also inefficient. When the ion exchange step and series of liquid-liquid extractions and precipitation are used in combination, the method is particularly inefficient and expensive to execute.
While the above publications focus on the removal of Cr, or the recovery of Cr and other metals with complicated and expensive processes, none of them are seen to disclose a method capable of selectively recovering at least one metal from a metal-bearing raw source material containing Cr in an efficient, relatively low cost manner.